In recent years the direct reduction of iron oxide to metallic iron has become a practical commercial reality with increasing worldwide acceptance and production. The direct reduced iron which results from direct reduction of iron oxide has a commercially demonstrated utility in iron and steelmaking and particularly in electric arc furnace steelmaking. The world demand today for direct reduced iron far exceeds the production capability of existing and planned direct reduction plants, due primarily to the dependence upon natural gas as the energy source in most of the direct reduction processes which have been commercially developed.
To satisfy the short-term and long-term demand for direct reduced iron, it is essential that new and improved direct reduction processes utilize solid fuels such as coal or lignite, to make direct reduction practical in those many countries such as the United States where large reserves of solid fuels are available and natural gas reserves are limited.
In recent years efforts to develop direct reduction processes which utilize solid fuels as the source of reductant for the reduction process have been directed primarily to two different basic types of processes. One type of process mixes solid fuel with iron oxide and processes the mixture in a rotary kiln, using air for combustion in the kiln. A typical example of this process is described in U.S. Pat. No. 3,046,106. The rotary kiln process has a fundamental thermal disadvantage in that approximately 65% of the solid fuel consumed by combustion is required to maintain the high temperature of the kiln, and only about 35% of the fuel value is available for reduction. This results in a solid fuel requirement in the range of 5.0 to 6.0 Giga calories per metric ton of direct reduced iron product.
A second type process gasifies the solid fuel in a separate combustion-type gasifier utilizing oxygen and steam for the gasification. The gas from the gasifier is then cooled and scrubbed, desulfurized, then utilized in a direct reduction furnace as the source of reductant. An example of this combination of gasifier and direct reduction furnace is described in U.S. Pat. No. 3,844,766. This combination also has a fundamental thermal disadvantage in that approximately 50% of the solid fuel is consumed by combustion in the gasifier and only the remaining 50% of the fuel value is available as a source of reductant. This combination, although highly efficient in the use of gas from the gasifier for reduction, requires approximately 4.0 to 5.0 Giga calories of solid fuel per metric ton of direct reduced iron.
In both types of solid fuel direct reduction processes described above, a major portion of the fuel value of the solid fuel is consumed by oxygen from air or from industrial oxygen. In the rotary kiln, only a minor portion is consumed by reaction with oxygen from the iron oxide being reduced.
French Patent 2,274,694 discloses a shaft furnace having a tubular reaction chamber through which the burden passes. The tube is externally heated by burners. The charge to the furnace is iron ore and coke with an optional desulphurizing agent. No provision is made handling the products of reaction nor for utilizing them in the process. The external heating of the tube of this type furnace creates a temperature gradient in the burden because the burden is not a good conductor of heat.
An electrically operated vertical shaft furnace is taught by U.S. Pat. No. 1,937,064 in which broken coke, graphite, silicon carbide or other conductors are charged to form a burden. Molten metal is then poured through the burden while electrical current also flows through the burden, thus refining the molten metal. The burden is a stationary granular mass of carbonaceous material which does not flow through the furnace. The burden also is not the material being treated, unlike the present invention.
Other patents which may be of interest to the reader include Carlsson U.S. Pat. No. 2,089,782, Brugger U.S. Pat. No. 2,755,325 and Southam U.S. Pat. No. 3,161,500.